1. Field of the Invention
The present invention relates to pellicles for masks used in photolithographic projection apparatus and methods of cleaning masks.
2. Description of the Related Art
The term “patterning device” as here employed should be broadly interpreted as referring to device that can be used to endow an incoming radiation beam with a patterned cross-section, corresponding to a pattern that is to be created in a target portion of the substrate. The term “light valve” can also be used in this context. Generally, the pattern will correspond to a particular functional layer in a device being created in the target portion, such as an integrated circuit or other device. An example of such a patterning device is a mask. The concept of a mask is well known in lithography, and it includes mask types such as binary, alternating phase shift, and attenuated phase shift, as well as various hybrid mask types. Placement of such a mask in the radiation beam causes selective transmission (in the case of a transmissive mask) or reflection (in the case of a reflective mask) of the radiation impinging on the mask, according to the pattern on the mask. In the case of a mask, the support structure will generally be a mask table, which ensures that the mask can be held at a desired position in the incoming radiation beam, and that it can be moved relative to the beam if so desired.
Another example of a patterning device is a programmable mirror array. One example of such an array is a matrix-addressable surface having a viscoelastic control layer and a reflective surface. The basic principle behind such an apparatus is that, for example, addressed areas of the reflective surface reflect incident light as diffracted light, whereas unaddressed areas reflect incident light as undiffracted light. Using an appropriate filter, the undiffracted light can be filtered out of the reflected beam, leaving only the diffracted light behind. In this manner, the beam becomes patterned according to the addressing pattern of the matrix addressable surface. An alternative embodiment of a programmable mirror array employs a matrix arrangement of tiny mirrors, each of which can be individually tilted about an axis by applying a suitable localized electric field, or by employing piezoelectric actuators. Once again, the mirrors are matrix addressable, such that addressed mirrors will reflect an incoming radiation beam in a different direction to unaddressed mirrors. In this manner, the reflected beam is patterned according to the addressing pattern of the matrix-addressable mirrors. The required matrix addressing can be performed using suitable electronics. In both of the situations described hereabove, the patterning device can comprise one or more programmable mirror arrays. More information on mirror arrays as here referred to can be seen, for example, from U. S. Pat. Nos. 5,296,891 and 5,523,193, and PCT publications WO 98/38597 and WO 98/33096. In the case of a programmable mirror array, the support structure may be embodied as a frame or table, for example, which may be fixed or movable as required.
Another example of a patterning device is a programmable LCD array. An example of such a construction is given in U.S. Pat. No. 5,229,872. As above, the support structure in this case may be embodied as a frame or table, for example, which may be fixed or movable as required.
For purposes of simplicity, the rest of this text may, at certain locations, specifically direct itself to examples involving a mask and mask table. However, the general principles discussed in such instances should be seen in the broader context of the patterning device as hereabove set forth.
Lithographic projection apparatus can be used, for example, in the manufacture of integrated circuits (ICs). In such a case, the patterning device may generate a circuit pattern corresponding to an individual layer of the IC, and this pattern can be imaged onto a target portion (e.g. comprising one or more dies) on a substrate (silicon wafer) that has been coated with a layer of radiation sensitive material (resist). In general, a single wafer will contain a whole network of adjacent target portions that are successively irradiated via the projection system, one at a time. In current apparatus, employing patterning by a mask on a mask table, a distinction can be made between two different types of machine. In one type of lithographic projection apparatus, each target portion is irradiated by exposing the entire mask pattern onto the target portion at once. Such an apparatus is commonly referred to as a wafer stepper. In an alternative apparatus, commonly referred to as a step and scan apparatus, each target portion is irradiated by progressively scanning the mask pattern under the projection beam in a given reference direction (the “scanning” direction) while synchronously scanning the substrate table parallel or anti-parallel to this direction. Since, in general, the projection system will have a magnification factor M (generally <1), the speed V at which the substrate table is scanned will be a factor M times that at which the mask table is scanned. More information with regard to lithographic devices as here described can be seen, for example, from U.S. Pat. No. 6,046,792.
In a known manufacturing process using a lithographic projection apparatus, a pattern (e.g. in a mask) is imaged onto a substrate that is at least partially covered by a layer of radiation sensitive material (resist). Prior to this imaging, the substrate may undergo various procedures, such as priming, resist coating and a soft bake. After exposure, the substrate may be subjected to other procedures, such as a post-exposure bake (PEB), development, a hard bake and measurement and/or inspection of the imaged features. This array of procedures is used as a basis to pattern an individual layer of a device, e.g. an IC. Such a patterned layer may then undergo various processes such as etching, ion-implantation (doping), metallization, oxidation, chemical, mechanical polishing, etc., all intended to finish off an individual layer. If several layers are required, then the whole procedure, or a variant thereof, will have to be repeated for each new layer. It is important to ensure that the overlay (juxtaposition) of the various stacked layers is as accurate as possible. For this purpose, a small reference mark is provided at one or more positions on the wafer, thus defining the origin of a coordinate system on the wafer. Using optical and electronic devices in combination with the substrate holder positioning device (referred to hereinafter as “alignment system”), this mark can then be relocated each time a new layer has to be juxtaposed on an existing layer, and can be used as an alignment reference. Eventually, an array of devices will be present on the substrate (wafer). These devices are then separated from one another by a technique such as dicing or sawing, whence the individual devices can be mounted on a carrier, connected to pins, etc. Further information regarding such processes can be obtained, for example, from the book “Microchip Fabrication: A Practical Guide to Semiconductor Processing”, Third Edition, by Peter van Zant, McGraw Hill Publishing Co., 1997, ISBN 0-07-0067250-4.
For the sake of simplicity, the projection system may hereinafter be referred to as the “lens.” However, this term should be broadly interpreted as encompassing various types of projection system, including refractive optics, reflective optics, and catadioptric systems, for example. The radiation system may also include components operating according to any of these design types for directing, shaping or controlling the projection beam of radiation, and such components may also be referred to below, collectively or singularly, as a “lens.” Further, the lithographic apparatus may be of a type having two or more substrate tables (and/or two or more mask tables). In such “multiple stage” devices the additional tables may be used in parallel or preparatory steps may be carried out on one or more tables while one or more other tables are being used for exposures. Dual stage lithographic apparatus are described, for example, in U.S. Pat. Nos. 5,969,441 and 6,262,796.
Referring to FIG. 2, a mask 10 used in a photolithographic projection apparatus typically includes a glass or quartz blank 11 having a patterned layer 12 of opaque material, for example chrome, formed on one surface. A pellicle 20 is provided to prevent contaminants, such as dust particles, from contacting the mask 10. Any contaminants on the mask 10 will alter the desired pattern to be imaged. The pellicle 20 includes a frame 21 attached to the blank 11 and a membrane 22 attached to the frame 21. The membrane 22 is positioned at a height H above the patterned layer that is larger than the focal length of the radiation imaged onto the mask 10 so as not to block radiation from reaching the mask. Any contaminants on the membrane 22 are also spaced above the mask 10 so as to be out of focus and not adversely affect the image of the pattern.
The membrane 22 may be formed by applying an anti-reflective coating to a fluoropolymer film or by spinning a polymer solution having a sufficient viscosity on a suitable film. The anti-reflective coating applied to the fluoropolymer film may be formed by spinning. The film must be relatively thick to withstand the forces associated with the spinning process. The thickness of the membrane 22 directly affects transmission of the radiation through the membrane 22 to the mask 10. Absorption and reflection of the radiation by the membrane 22 reduce the transmission of radiation to the mask 10 and prevent all of the radiation from being used in the photolithographic process. The membrane 22 must be thick enough to have mechanical strength sufficient to be spin coated, lifted and adhesively mounted to the frame 21. The pellicle 20 shown in FIG. 2, including the polymer film and spin coated coating, is generally referred to as a soft pellicle to distinguish from a quartz pellicle, which is generally referred to as a hard pellicle. Hard pellicles are generally more expensive than soft pellicles and may act as an additional optical element, which may adversely affect the imaging and overlay performance of photolithographic projection apparatus. Soft pellicles, although less expensive to manufacture, can introduce optical distortions due to bending or sagging of the membrane. Soft pellicles are also less durable than hard pellicles and may require replacement more frequently than hard pellicles.
The membrane 22 is generally fragile and easily destroyed by conventional mask cleaning processes. Conventional cleaning processes may include spraying a cleaning fluid, for example de-ionized water or ammonium hydroxide, on the mask 10, spinning the mask 10 to remove excess cleaning fluid, and a rinse spray. The membrane 22 is often removed prior to cleaning the mask and then reattached to the frame 21. The mask 10 must then be requalified for use in a photolithographic projection apparatus. Each pellicle is built to match a particular mask. The process of removing the membrane 22, cleaning the mask 20, reattaching the membrane 22 to the frame 21 and requalifying the mask 10 is time consuming and costly. Nonuniformities in the thickness and roughness of the pellicle membrane also cause nonuniformities in the membrane's transmission of the radiation. Film thickness must be precisely controlled to allow operation at the fringe maxima for the radiation wavelength.
The trend toward smaller integrated circuit devices requires lithographic projection apparatus that can print patterns having features of even smaller critical dimensions (CD) than those currently printed using 248 nm and 193 nm radiation. Lithographic projection apparatus utilizing 157 nm radiation are currently being developed to print pattern features having CD's as small as 70-100 nm. However, known polymers currently used for pellicle membranes in 248 nm and 193 nm photolithography are not suitable for use in 157 nm photolithography. Commercially available fluoropolymers, such as TEFLON® AF and CYTOP®, rapidly burst under irradiation by 157 nm radiation because they lack sufficient mechanical integrity.
Fluoropolymers currently being developed have sufficient transparency to produce transmission rates above 95%. Upon irradiation the fluoropolymers undergo photochemical darkening, which reduces the transmission rate and the useful life of the pellicle membrane. It is generally assumed that the useful life of the pellicle increases uniformly with increasing transparency. However, TEFLON® AF (TAFx) polymers developed by DuPont for use as pellicles in 157 nm photolithography have shown that materials with different absorptions have similar useful lifetimes and polymers with similar absorptions have different useful lifetimes. Ideally, a pellicle for use in 157 nm photolithography should be at least 98% transparent and resist photochemical darkening to remain useful for an exposure lifetime of 7.5 kJ/cm2.
It is important that fluoropolymers used as pellicles for 157 nm photolithography have the required optical properties (i.e., transparency and absorption), film formation characteristics and mechanical and photochemical radiation durability. The fluoropolymers must also have low optical absorptions necessary to produce minimal outgassing and be compatible with noncontaminating adhesives used to attach the membrane to the pellicle frame, the gasket material used to attach the pellicle frame to the mask and the material of the pellicle frame. Because optical absorption caused by air is four orders of magnitude higher at 157 nm than at 193 nm, the entire exposure system needs to be designed and maintained contaminant free. The optical path, including the wafer and mask stages, can be exposed to only part per million concentrations of oxygen, water and organic molecules. An additional molecular cleaning step is needed before the mask is exposed. Current mask cleaning techniques include purging with gas, for example nitrogen. The purging process increases production cost and time of integrated circuit devices produced using photolithographic projection apparatus.